Softening of hard water by the removal of calcium values therefrom is required for both industrial and household use to decrease scale formation in equipment such as boilers, hot water pipes and appliances such as pots and kettles, and also for the decrease of detergent consumption in household machines such as dishwashers and laundry washing machines. Softening of water is, moreover, effected as a pretreatment for water desalination.
The factors to be considered in the choice of a water softening process include the raw water quality, the end use and desired quality of the soft water, the ways and costs of disposing the waste streams, ecological problems associated with the process in general and with the waste disposal in particular, and the versatility of the process and its adaptability to different processing scales.
Known water softening processes proceed either by way of ion-exchange or by way of precipitation. In the ion-exchange processes the scale forming Ca.sup.2+ and Mg.sup.2+ ions are exchanged for Na.sup.+ and regeneration of the ion-exchange resin is achieved with a large excess of NaCl, with the regeneration effluent being a relatively concentrated aqueous solution of sodium chloride which has to be disposed. Consequently, by this method considerable amounts of sodium salts are introduced into the sewage and finally into the ground water which is ecologically undesirable. Alternatively, it is possible to use weak acid resins which exchange H.sup.+ for Ca.sup.2+ and Mg.sup.2+ and to regenerate the spent resins with a weak acid. While this method gives rise to less pollution and is thus more attractive, it is more expensive and has the further disadvantage of yielding acidic soft water which is corrosive, and acidic effluent which for obvious reasons gives rise to ecological problems.
Precipitation has traditionally been carried out by the lime soda process in which slaked lime is added to hard water to convert water soluble calcium bicarbonate into water insoluble calcium carbonate. This process results in a sludge of high water content which is difficult to filter and requires cumbersome treatment.
According to U.S. Pat. No. 3,976,569 hard water is softened by pH controlled, selective precipitation of CaCO.sub.3 to form a thin layer thereof, followed by a raise of the pH whereby magnesium hydroxide is precipitated. The resulting slurry is filtered through the first formed CaCO.sub.3 layer and a satisfactory degree of softening is reported. The object of that process is to improve the filtering of the precipitated Mg(OH).sub.2, the main disadvantage being the long time requirement for completion of the precipitation which in several batch process runs varied between 19 to 22 hours and which obviously imposes long residence times.
According to a publication by A. Graveland in Aqu. 2, 80, the rate of precipitation of calcium carbonate from hard water by the addition of sodium hydroxide can be significantly improved by carrying out the crystallisation on sand or marble grains in a fluidised bed. By this method the precipitation time is drastically reduced, the residence time in the reactor being approximately 5 minutes with upward flow velocities of the water across the bed ranging between 40 and 150 m/hr. However, in the performance of this process large CaCO.sub.3 grains are formed and consequently the fluidised bed forming material has to be replaced from time to time. Moreover, large size installations are required and for both these reasons this method is intrinsically unsuitable for small scale water softening operations.
It is the object of the present invention to provide an improved process for the softening of hard water by precipitation, operable both on a large, industrial and municipal scale and small, domestic scale.